NASA's Neil Gehrels Swift Observatory began tracking the 2016 event, named GRB160821B, minutes after it was detected.

The early catch enabled the research team to gather new insights that were missing from the kilonova observations of the LIGO event, which did not begin until nearly 12 hours after the initial collision.

Lead author Eleonora Troja, ‘The 2016 event was very exciting at first. It was nearby and visible with every major telescope, including NASA's Hubble Space Telescope. But it didn't match our predictions--we expected to see the infrared emission become brighter and brighter over several weeks.

‘Ten days after the event, barely any signal remained. We were all so disappointed. Then, a year later, the LIGO event happened.

‘We looked at our old data with new eyes and realized we had indeed caught a kilonova in 2016. It was a nearly perfect match. The infrared data for both events have similar luminosities and exactly the same time scale.’